The present invention relates to an air control device for an internal combustion engine, and more particularly to a supporting structure of a throttle valve in such an air control device including a throttle body, a throttle shaft for mounting the throttle valve and a pair of ball bearings for rotatably supporting the throttle shaft.
Such a throttle valve supporting structure is disclosed, for example, in Japanese Patent Laid-open Publication No. 62-129538 (which will be hereinafter referred to as a first reference) and Japanese Utility Model Publication No. 60-26201 (which will be hereinafter referred to as a second reference).
In the throttle valve supporting structure as shown in the first reference, inner rings of first and second bearings axially abut against stopper portions provided on a throttle shaft, and outer rings of the first and second bearings axially abut against engagement portions formed with a throttle body. Further, a spring such as a wave washer is interposed between the outer ring of at least one of the ball bearings and the corresponding engagement portion. The inner circumferential surfaces of the inner rings of the first and second ball bearings are engaged by clearance fit with the throttle shaft, and the outer circumferential surfaces of the outer rings are also engaged by clearance fit with the inner circumferential surfaces of the engagement portions.
On the other hand, in the throttle valve supporting structure as shown in the second reference, inner rings of first and second ball bearings are engaged by clearance fit with the throttle shaft, and outer rings of the first and second ball bearings are engaged by interference fit with the throttle body.
Generally, the throttle body is mounted directly or indirectly to the internal combustion engine, and it receives vibration of the engine. In the first reference, pressure is axially applied to the inner and outer rings by the spring so as to axially fix the throttle shaft and the throttle valve to the throttle body. However, since there are defined the clearances between the outer rings and the throttle body, the ball bearings and the throttle body are independently vibrated to cause the generation of wear between the throttle body and the ball bearings. Further, this causes another problem such that an axial clearance defined between the throttle valve and an inner wall surface of a suction passage formed in the throttle body is changed to render air control unstable. Additionally, there is also generated the wear between the throttle valve and the inner wall surface of the suction passage and between the throttle shaft and insert holes formed in the throttle body for inserting the throttle shaft, which also renders the air control unstable. Although this problem may be solved to some extent by increasing the pressure of the spring, friction of the ball bearings will be increased in this case to unsmoothen the rotation of the ball bearings.
In the second reference, since the inner rings of the first and second ball bearings are engaged by clearance fit with the throttle shaft, there tends to be generated fretting wear between the throttle shaft and each inner ring to render the air control unstable. Generally, the throttle shaft is formed of carbon steel, and the inner ring is formed of quenched bearing steel. Therefore, the inner ring is harder than the throttle shaft to cause the generation of fretting wear on the throttle shaft. Recently, there has been developed a high-speed engine which will accompany a large vibration. Accordingly, it is considered important to suppress the fretting wear.
The fretting wear is considered to be suppressed by plating hard chromium, for example, onto the surface of the throttle shaft or quenching the throttle shaft to increase a hardness thereof. However, the plating method causes a large change in diameter of the throttle shaft to reduce a dimensional accuracy thereof. On the other hand, the quenching method causes the generation of strain of the throttle shaft. Thus, both the methods have a disadvantage in air controllability.
As a further method, the throttle shaft may be formed of alloy steel having a large hardness. However, such a material is high in material cost and machining cost due to bad machinability.